Thermal processor and method for using the same

The present invention is directed to a thermal development apparatus and a method of using the same to thermally develop photosensitive relief image printing elements.

Flexography is a method of printing that is commonly used for high-volume runs and is employed for printing on a variety of substrates such as paper, paperboard stock, corrugated board, films, foils and laminates. Newspapers and grocery bags are prominent examples. Coarse surfaces and stretch films can be economically printed only by means of flexography. Flexographic printing plates are relief plates with image elements raised above open areas. Such plates offer a number of advantages to the printer, based chiefly on their durability and the case with which they can be made. Although photopolymer printing elements are typically used in “flat” sheet form, there are applications that utilize the printing element in a continuous cylindrical form, as a continuous in-the-round (CITR) photopolymer sleeve.

A typical flexographic printing blank as delivered by its manufacturer is a multilayered article made of, in order, a backing or support layer, one or more unexposed photosensitive or photocurable layers, a protective layer, slip film and/or laser ablatable layer, and a cover sheet.

The flexographic relief image printing element is generally produced from a photosensitive printing blank by imaging the photosensitive printing blank to produce a relief image on the surface of the photosensitive printing element. This can be accomplished by selectively exposing the photosensitive material to actinic radiation, which exposure acts to harden or crosslink the photosensitive material in the irradiated areas. The photosensitive printing blank contains one or more layers of uncured photosensitive material on a suitable backing layer. The photosensitive printing blank can be in the form of a flat, planar plate that is mounted on a carrier sleeve or a continuous (seamless) sleeve.

The photosensitive relief image printing element may be selectively exposed to actinic radiation in one of three related ways. In a first alternative, a photographic negative with transparent areas and substantially opaque areas is used to selectively block the transmission of actinic radiation to the printing element. In a second alternative, the photopolymer layer is coated with an actinic radiation (substantially) opaque layer that is sensitive to laser ablation. A laser is then used to ablate selected areas of the actinic radiation opaque layer creating an in situ negative. This technique is well-known in the art, and is described for example in U.S. Pat. Nos. 5,262,275 and 6,238,837 to Fan, and in U.S. Pat. No. 5,925,500 to Yang et al., the subject matter of each of which is herein incorporated by reference in its entirety. In a third alternative, a focused beam of actinic radiation is used to selectively expose the photopolymer. Any of these methods is acceptable, with the criteria being the ability to selectively expose one or more layers of photosensitive material to actinic radiation thereby selectively curing portions of the one or more layers of photosensitive material.

Thereafter, the one more layers of photosensitive material are developed to remove uncured (i.e., non-crosslinked) portions of the one or more layers, without disturbing the cured portions of the one or more layers of photocurable or photosensitive material, to produce the relief image. The development step can be accomplished in a variety of ways, including water washing, solvent washing, and thermal development.

Upon completion of the development step, the relief image printing element may be subjected to one or additional steps, including, for example, post-exposure to further actinic radiation and/or detackification, and is then cooled and is ready to use.

It is an object of the present invention to improve quality of photosensitive printing elements processed using thermal development.

It is another object of the present invention to reducing machine processing issues in a thermal development apparatus that can adversely affect the quality of the photosensitive relief image printing elements processed therein.

It is still another object of the present invention to improve temperature control during thermal development.

It is another object of the present invention to reduce temperature variations of the photosensitive printing element during thermal development.

It is still another object of the present invention to control the temperature of the photosensitive printing element at entry or reentry of the photosensitive printing element into the thermal development apparatus.

It is another object of the present invention to provide a means of gradually heating the photosensitive printing element so that the temperature of the photosensitive printing element approaches the processing temperature of the thermal development apparatus.

To that end, in one embodiment, the present invention relates generally to a thermal development apparatus for forming a relief structure on a photosensitive printing element, wherein the photosensitive printing element comprises a flexible substrate and at least one layer of photosensitive material comprising cured portions of photosensitive material and uncured portions of photosensitive material on the flexible substrate, the apparatus comprising:

The present invention also relates generally to a method of thermally developing photosensitive relief image printing elements using the apparatus described herein.

Also, while all elements may not be labeled in each figure, all elements with the same reference number indicate similar or identical parts.

The present invention relates to an improved thermal development apparatus and an improved method of using the apparatus to remove non-crosslinked polymer from an imaged and exposed surface of a relief image printing element during a process for manufacturing the flexographic relief image printing element.

Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. It will be further understood that terms used herein should be interpreted as having a meaning that is consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

As used herein, “a,” “an,” and “the” refer to both singular and plural referents unless the context clearly dictates otherwise.

As used herein, the terms “comprises,” “comprising,” “includes” and/or “including” specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

As used herein, the term “about” refers to a measurable value such as a parameter, an amount, a temporal duration, and the like and is meant to include variations of +/−15% or less, preferably variations of +/−10% or less, more preferably variations of +/−5% or less, even more preferably variations of +/−1% or less, and still more preferably variations of +/−0.1% or less of and from the particularly recited value, in so far as such variations are appropriate to perform herein. Furthermore, it is also to be understood that the value to which the modifier “about” refers is itself specifically disclosed herein.

Relative terms such as “below” or “above” or “upper” or “lower” or “horizontal” or “vertical” may be used herein to describe a relationship of one element, layer, or region to another element, layer, or region as illustrated in the Figures. It will be understood that these terms and those discussed above are intended to encompass different orientations of the device in addition to the orientation depicted in the Figures.

It will be understood that when an element such as a layer, region, or substrate is referred to as being “on” or extending “onto” another element, it can be directly on or extend directly onto the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” or extending “directly onto” another element, there are no intervening elements present. Likewise, it will be understood that when an element such as a layer, region, or substrate is referred to as being “over” or extending “over” another element, it can be directly over or extend directly over the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly over” or extending “directly over” another element, there are no intervening elements present. It will also be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present.

As used herein the term “substantially-free” or “essentially-free” if not otherwise defined herein for a particular element or compound means that a given element or compound is not detectable by ordinary analytical means that are well known to those skilled in the art of metal plating for bath analysis. Such methods typically include atomic absorption spectrometry, titration, UV-Vis analysis, secondary ion mass spectrometry, and other commonly available analytically methods.

During thermal development, an imagewise exposed photosensitive printing element is developed using heat and the differential melting temperature between cured and uncured photopolymer is used to develop the latent image. The basic parameters of this process are known, as described in U.S. Pat. Nos. 5,279,697, 5,175,072 and 3,264,103, in published U.S. patent publication Nos. US2003/0180655 and US2003/0211423, and in WO01/88615, WO01/18604, and EP1239329, the teachings of each of which are incorporated herein by reference in their entirety. As described herein, a photosensitive printing blank containing one or more layers of photosensitive material is selectively exposed to actinic radiation, resulting in portions of the one or more layers of photosensitive material being crosslinked and cured while other portions of the one or more layers of photosensitive material remain uncrosslinked and uncured, which uncured portions may be removed during the development step to reveal to relief image.

In order for the photosensitive printing element to be thermally developable, the composition of the one or more layers of photosensitive material must be such that there exists a substantial difference in the melt temperature between the cured and uncured photosensitive material. It is precisely this difference that allows the creation of an image in the one or more layers of photosensitive material when heated. The uncured photosensitive material (i.e., the portions of the one or more layers of photosensitive material not contacted with actinic radiation) melts and/or substantially softens while the cured photosensitive material remains solid and intact at the temperature chosen. Thus, the difference in melt temperature allows the uncured photosensitive material to be selectively removed, thereby creating the desired image.

Thereafter, uncured photosensitive material can be softened and/or melted and subsequently removed. In most instances, the heated printing element is contacted with an absorbent material that absorbs or otherwise removes the softened and/or melted uncured photosensitive material. This removal process is generally referred to as “blotting.”

depicts a typical thermal development apparatusfor developing a relief image on a photosensitive printing element. The thermal development apparatuscomprises an enclosurefor housing the elements of the thermal development apparatus.

The thermal development apparatusaccepts a previously formed and imagewise actinic radiation exposed photosensitive printing element. In one embodiment, the photosensitive printing element may be backed with a resilient compressible layer (not shown) as described, for example in U.S. Pat. No. 9,069,255 to Hennessey et al., the subject matter of which is herein incorporated by reference in its entirety. The photosensitive printing elementtypically includes a flexible substrate (i.e., backing layer) and at least one layer of photosensitive material comprising cured portions of photosensitive material and uncured portions of photosensitive material disposed on the flexible substrate. In addition, in an in situ mask, formed from an infrared sensitive layer, may be disposed on the at least one layer of photosensitive material, which in situ mask is formed during the imagewise exposure step prior to development and which is also removable during thermal development. Examples of suitable photosensitive printing elements are described in U.S. Pat. No. 5,175,072 to Martens, U.S. Pat. Nos. 5,262,275 and 6,238,837 to Fan, and U.S. Pat. Nos. 5,925,500 and 6,605,410 to Yang et al., the subject matter of each of which is herein incorporated by reference in its entirety.

Prior to thermal development, a portion of the at least one layer of photosensitive material may be cured by actinic radiation through the lower surface of the flexible substrate to form a cured “floor” that sets the depth of plate relief. Next, the film is imagewise exposed to actinic radiation from the opposite surface to cure the desired portions of the at least one layer of photosensitive material, preferably through the in situ mask. Thus, after curing, the at least one layer of photosensitive material comprises cured portions and uncured portions, and the uncured portions must be removed during the development step.

A conveyorattached to a drive motor (not shown) is used to transport and convey the photosensitive printing elementthrough the thermal plate processing system. The conveyoris mounted in a fixed position in the enclosure, and comprises a continuous loop support meanssupported by at least a first rollerand a second roller. Optionally, one or more additional rollers (not shown) may be used to provide additional support to the conveyorand prevent the continuous loopfrom sagging from the weight of the photosensitive printing element. The continuous loop support meansmay comprise a wire mesh. Alternatively, the support means may be a rotatable drum.

The photosensitive printing elementis generally held in place against the continuous loopof the conveyorby fastening means, which fastening means may comprise a clampthat secures a leading edge of the photosensitive printing element. The photosensitive printing elementmay also be held in place by vacuum (not shown). The vacuum may be provided through the wire mesh of the continuous loop support meansand may also be provided to at least one of the first rollerand the second rollerof the conveyor, and used, alone or in combination with fastening means, to hold the photosensitive printing elementin place on the continuous loopof the conveyor.

During operation, the conveyorwith photosensitive printing elementmoves in a first directiontowards heatable rollersuch that the photosensitive printing elementpasses through a gapbetween the conveyorand the heatable rolleras the continuous loopof conveyorrotates over and around the first roller. Heatable rollerrotates in an opposite directionfrom the conveyor. Heatable rolleris capable of being urged towards the photosensitive printing elementpositioned on the conveyoras the conveyor moves in first directionand heatable rollermoves in an opposite direction. Preferably, the heatable rolleris fixably mounted on a pivot (not shown), which allows it to be urged towards the conveyor.

The heatable rolleris typically urged toward the photosensitive printing elementon the conveyorusing suitable means, such as one or more pneumatic cylinders. The pneumatic cylinder(s)positions the heatable rollerat a preset distance from the outer surface of the first rollerof the conveyorto produce the gapthrough which the photosensitive printing elementpasses as it travels on the continuous loopof the conveyoraround the first roller.

A web of absorbent materialis supplied over at least a portion of an outer surfaceof the heatable roller. The web of absorbent materialis capable of absorbing (removing) material that is liquefied or softened from the photosensitive printing elementwhen the heatable rollerrotates and is heated and the web of absorbent materialcontacts at least a portion of the photosensitive printing element. The heatable rollerrotates in a directionopposite to the directionof the conveyorso that the photosensitive printing elementand the web of absorbent materialcan be contacted with each other and then separated.

The pneumatic cylinderis controlled to adjust the gapdepending on the thickness of the photosensitive printing element. The pneumatic cylinder(s)causes photosensitive printing elementand the web of absorbent materialto come into contact at the gapbetween the conveyorand the heatable rolleras the conveyorrotates in a first directionand the heatable rollerrotates in an opposite directionsuch that at least a portion of the liquefied or softened material is absorbed by the web of absorbent material.

Heat may be provided to the heatable rollerby a core heater that is capable of maintaining a skin temperature of the heatable rollerthat will soften or liquefy at least a portion of the photosensitive material. The temperature to which the heatable rolleris heated is chosen based on the composition of the photosensitive material and is based on the melting temperature of the monomers and polymers contained within the photosensitive material. While the heatable rollerpreferably comprises an electrical core heater to provide the desired skin temperature, the use of steam, oil, hot air, and a variety of other heating sources may also provide the desired skin temperature.

The web of absorbent materialis supplied to at least the portion of the outer surface of the heatable rollerfrom a supply rollof the web of absorbent material. The selection of the absorbent materialdepends in part upon the thickness of the photosensitive printing elementto be processed, the melting temperature of the web of absorbent material, and the heat transfer characteristics of both the photosensitive printing elementand the web of absorbent material.

The web of absorbent materialcomes into face-to-face contact with the heatable roller, which may be heated to and operated at a temperature within the range of about 100° C. to about 250° C., more preferably between about 120° C. and about 200° C. The upper limit is determined in large part by the melting temperature of the web of absorbent material. The temperature of the heatable rollermust also be low enough so that when the web of absorbent materialis not moving and the portions of the web of absorbent materialcontacting the heatable rollerare at rest, the absorbent materialdoes not melt. Suitable means for maintaining uniform tension in the web of absorbent material throughout the system may be used, including for example, one or more idler rollers (not shown). Other means for maintaining tension in the web may also be provided and would be known to those skilled in the art.

It is also critical that the linear speed of the heatable roller, the web of absorbent material, and the photosensitive printing elementbe substantially the same to avoid any shear stress on the photosensitive printing element, which stress is known to cause uneven relief portion plate thickness.

A take-up rollermay be provided to wind the web of absorbent materialafter processing through the thermal development apparatus. If present, the take-up rolleris independently belt driven by a motor, which is preferably a variable speed motor. The take-up rollercollects the web of absorbent materialafter it has contacted the photosensitive printing elementand removed portions of the photosensitive material that were liquefied or softened. The speed of the motoris adjusted so as to not interfere with the selected web tension. If the motor interferes with web tension, the resulting flexographic plate could potentially have variable heights in the relief portions or might warp and be commercially unacceptable.

The systemmay also include external heating meanspositioned adjacent to a pointwhere the web of absorbent materialcontacts the photosensitive printing elementon the conveyor. The external heating meansprovide a heat source to further soften and liquefy portions of one or layers of the photosensitive printing elementimmediately prior to the point where the web of absorbent materialis brought into contact with the photosensitive printing element.

The conveyor, including first rollerand second rolleras well as the heatable rollerare driven by suitable means, i.e., a motor. In addition, a controller, such as a microprocessor, may be used to control the operation of each of the elements in the thermal development apparatus. Such controllers are well known in the art. One example of a controller used to control the various elements in a plate processor is described in U.S. Pat. No. 5,279,697 to Peterson et al., the subject matter of which is herein incorporated by reference in its entirety.

In addition, a rotating drum (not shown) may be used in place of conveyorto bring the photosensitive printing elementinto contact with the web of absorbent material at the nip. Thus, while the present invention is described with respect to conveyor, a rotating drum can be used in place of the conveyor if desired and, if used, would produce a similar result.

The thermal development apparatusmay also have ventilation means (not shown) connected to the enclosureas described, for example, in U.S. Pat. No. 7,044,055 to Gotsick et al., the subject matter of which is herein incorporated by reference in its entirety. These means are operative in removing volatile organic compounds and other contaminants from the enclosurefor subsequent treatment and disposal.

The inventors of the present invention have also discovered that improved temperature control of the photosensitive printing elementitself, rather than simply setting a temperature of a heating element or other part of the apparatus, can provide a more consistent relief image printing plate product. In other words, a more consistent control of the temperature of the photosensitive relief image printing element being processed and especially a more consistent control of the temperature of the photosensitive relief image printing element, to regulate the temperature that more closely monitors the heating roll, can reduce the risk of machine processing issues and increase the quality of the resulting photosensitive relief image printing element.

The inventors of the present invention have discovered that when the photosensitive printing elementis at a temperature that is much lower than the temperature at the point at which the heatable rolleris brought into contact with the photosensitive printing elementat the nip, the temperature variance can lead to plate quality issues with the surface of the printing plate and processing issues within the thermal development apparatus. Thus, raising the temperature of the photosensitive printing elementto a temperature that approaches the operating temperature of the heatable rollercan result in an improved product.

As described herein, a heating zone is generally created within photosensitive printing element. When the surface of the photosensitive printing elementis maintained at a colder temperature than the heatable rollerat the processing nip, this temperature variance can lead to plate quality issues with the surface of the photosensitive printing elementand processing issues in the thermal development apparatus.

As described above, temperature control has generally been handled using a main heating source, which generally comprises the heatable roller, and may include a secondary heat source (i.e., external heating means) which generally constitutes an infrared lamp positioned directly in front of the processing nip.

The inventors of the present invention have found that processing of the photosensitive printing elementin the thermal development apparatuscan be further optimized to achieve better stability and better plate quality.